Although Zack Allan is said to hold the rank of "Sergeant" (prior to his promotion) at that time he was a peace officer employed by the Earth Alliance though Babylon 5 funding and as such not a member of the Eathforce military. This is why the grey security uniform worn by Zack and the other security officers lacked the stylised 'EA' pin worn by members of Earthforce proper and had a 'B5' shoulder patch instead of an Earthforce division badge.